As well known, a rotating machine such as a centrifugal compressor is configured to allow gas to pass through a rotating impeller in a radial direction of the rotating impeller, and to compress the gas by centrifugal force occurring during rotation. A multistage centrifugal compressor including impellers of multiple stages in an axial direction, and compressing gas in stages is known as this type of centrifugal compressor.
The impellers are rotatably supported by a rotating shaft in a casing of the centrifugal compressor. The centrifugal compressor suctions a fluid such as air or gas via a suction port of the casing, and applies centrifugal force to the fluid by rotating the impellers via the rotating shaft. Kinetic energy induced by the centrifugal force is converted into compression energy by diffusers and a scroll portion, and the compressed gas is sent from a discharge port of the casing.
Among the aforementioned rotating machines, particularly, a centrifugal compressor, in which many impellers are installed on the same shaft, and each of the impellers has one outlet for gas, is referred to as a straight type centrifugal compressor among single-shaft multistage centrifugal compressors.
PTL 1 discloses an example of a single-shaft multistage centrifugal compressor in which a stage having a vaneless diffuser and a stage having a vaned diffuser are combined together. This centrifugal compressor aims to maintain high efficiency in the stage having the vaned diffuser, and to secure a wide operating range in the stage having the vaneless diffuser.